Landing gear noise reduction

ABSTRACT

An aircraft landing gear ( 1 ) is described including a noise-reducing element that includes an air-deflecting surface, in the form of a fairing ( 4 ), and a perforated airflow-reducing region ( 8 ). In use, some air passes through the perforations, whilst other air is deflected by the fairing away from a noise-generating region of the landing gear. Noise caused by the passing of the landing gear through the air is therefore reduced.

BACKGROUND OF THE INVENTION

The present invention relates to landing gear. In particular, theinvention relates to landing gear designed to reduce the noise generatedby the interaction of the landing gear and the air flowing past itduring flight, take-off and/or landing.

It is desirous to minimise the noise generated by aircraft, for example,to lessen disruption or inconvenience, resulting from aircraft noise,caused to the public on the ground near airports. The interaction of thelanding gear and the air flowing past it results in turbulent flows andconsequently noise. A significant amount of noise is generated in thisway.

SUMMARY OF THE INVENTION

It is an aim of the present invention to provide a landing gear thatgenerates less noise compared to a landing gear of the same size thathas not been adapted, designed or manufactured in accordance with thepresent invention.

According to the invention there is provided an aircraft landing gearincluding a noise-reducing element that includes an air-deflectingsurface and an airflow-reducing region through which, in use, air maypass, whereby the noise-reducing element is able in use to deflect airaway from a noise-generating region of the landing gear, whilst allowingsome air to pass through the element, thereby reducing the noise causedby the passing of the landing gear through the air.

Thus noise caused by the interaction of the air and the landing gearcaused during the approach when landing may be reduced by means ofproviding a noise-reducing element according to the present invention.The provision of an airflow-reducing region enables the noise-reducingeffect of the noise-reducing element to be improved. Exactly how theprovision of such an airflow-reducing region reduces noise has not beenfully analysed, although a possible explanation relating to thereduction of the production of turbulent airflows is provided below.

We have ascertained that certain parts of the landing gear contributegreatly to the noise generated by the landing gear as it passes throughthe air. Deflecting air away from these parts reduces noise generated bysuch parts, but can result in separate air flows being created thatconverge downstream of the landing gear. The converging air flowsinteract with each other, possibly resulting in turbulent air flows thatgenerate extra noise. Also, the provision of an element that deflectsair, may have a shape that results in eddy current(s) and furtherturbulence being created immediately upstream of the element. Providingan airflow-reducing region may, for example, divert a portion of theairflow that would otherwise contribute to such turbulent airflows.Allowing some air to flow, or bleed, through the element via theairflow-reducing region may therefore further reduce noise that mightotherwise be generated, despite there being noise generated by theinteraction of the element and the air flowing through theairflow-reducing region. Thus, the noise-reducing element isadvantageously configured to reduce, in use, the amount of turbulentairflow generated in the region of the landing gear.

An alternative or additional means by which the present invention mightreduce noise may arise when the air deflected by the air-deflectingsurface is caused to flow downstream onto or past other components ofthe landing gear or aircraft. In such a case, providing anairflow-reducing region may reduce the noise generated by theinteraction of the high speed deflected airflows with such otherdownstream components, for example, by reducing the amount of air, or byreducing the speed of the air, flowing past or onto such othercomponents.

The airflow-reducing region may, for example, therefore be considered asan airflow-bleeding region or even an airflow-bypass region in that someof (i.e. a portion and not all) the air that would otherwise bedeflected by the noise-reducing element is allowed to flow through theelement.

The noise-reducing element may also be configured to streamline, in use,the flow of air past the landing gear.

The airflow-reducing region may reduce the airflow that is deflected bythe noise-reducing element by means of one or more appropriatelypositioned and shaped apertures in the element. The airflow-reducingregion advantageously includes a multiplicity of apertures throughwhich, in use, air may pass. It is believed that providing amultiplicity of apertures of a given sum area provides a greaternoise-reduction effect than a single round aperture of the same areacould. For example, a single round aperture of a given area might resultin an air flow that interacts with a part of the landing gear to causeturbulent flows downstream, whereas twenty apertures each having an areaequal to a twentieth of the area of the single hole would result in theair flow being provided over a greater area, which it is thought reducesthe possibility of unwanted noise being generated. The noise-reducingelement preferably includes at least 10 apertures, more preferablyincludes more than 20 apertures and even more preferably has more than50 apertures. The apertures may be in the form of perforations.

The arrangement of the apertures across the air-deflecting surface maybe non-uniform. It will of course be understood that the arrangement ofthe apertures as a whole may be non-uniform or irregular whilst at leastone substantial portion of the air-deflecting surface has a regular oruniform arrangement of apertures. The non-uniformity of the aperturesmay merely be as a result of the apertures not being distributed evenlyacross the air-deflecting surface. For example, the air-deflectingsurface may include a centre portion having a uniform arrangement ofapertures and a peripheral portion having no apertures, such that thearrangement of the apertures across the air-deflecting surface as awhole is non-uniform. Furthermore, there may be one or more areas havingno apertures, each area being disposed between regions having apertures.Such areas (having no apertures) may for example be needed in positionswhere the noise-reducing element has a structural function (for examplethere may be flanges or stiffeners on the rear surface of thenoise-reducing element), where the provision of apertures would not bedesirable.

The air-deflecting surface may include a first region encompassing noapertures and a second region encompassing at least ten apertures, thearea covered by the first region having a minimum dimension that is atleast as great as the maximum dimension of the area covered by thesecond region. The second region may for example be in or near themiddle of the air-deflecting surface. The first region may for examplebe near to the periphery of the air-deflecting surface.

Preferably the airflow-reducing region is disposed between two regionsdefined by the air-deflecting surface. The airflow-reducing region ispreferably surrounded on all sides by the rest of the air-deflectingsurface. The airflow-reducing region may thus not extend to the edge ofthe air-deflecting surface.

There may be more than one airflow-reducing region on the noise reducingelement. There may for example be a plurality of discreteairflow-reducing regions. Such discrete airflow-reducing regions couldof course be considered as being separate sub-regions of a singleairflow-reducing region.

Similarly, the air-deflecting surface may comprise separate discretesurfaces that together form a single, albeit with discontinuities,air-deflecting surface.

The or each aperture is preferably round in cross-section. Other shapescould of course be used, but round holes are easily machined.

The or each aperture is preferably formed such that the portion definingthe part of the aperture on the surface that in use faces the airflow(i.e. the upstream surface) has substantially no sharp edges. Sharpedges might, under certain conditions, generate extra noise. Preferably,that portion has substantially no edges defined by surfaces meeting atangles of 90° or less. The or each aperture is preferably countersunk onthe surface that in use faces the airflow (i.e. the upstream surface).

The passageway through the noise-reducing element defined by eachaperture preferably flares out towards the surface that in use faces theairflow (i.e. the upstream surface). It is preferred that the passagewayalso has a portion of substantially constant cross-sectional area.

The airflow-reducing region may have a volume of free space permittingthe flow of air through the noise-reducing element and a volume of solidmaterial defining the volume of free space. The airflow-reducing regionmay be considered as having a porosity. For example, the porosity may bedefined as the percentage of free space to the total volume occupied bythe airflow-reducing region. The porosity of the airflow-reducing regionis preferably in the range from 10% to 60% and more preferably between20% and 50%.

Advantageously, the sum of the cross-sectional area of all of theapertures (at their narrowest) in the airflow reducing region is equalto a percentage (hereinafter the perforation percentage) in the rangefrom 10% to 60% of the total area of the airflow-reducing region.Preferably, the average width of the air-deflecting surface betweenadjacent apertures is wider than the average minimum dimension of theapertures. More preferably, the perforation percentage is between 20%and 50%. Yet more preferably, the perforation percentage is in the rangefrom 40% to 45% (inclusive), and even more preferably is in the rangefrom 42% to 44% (inclusive). In an embodiment described below theperforation percentage is about 44%.

The perforation percentage is preferably chosen so that, in use atnormal speeds on approach when landing, the relative speed of the airimmediately behind the airflow-reducing region is between 20% and 80% ofthe relative air speed in front of the airflow-reducing region. Theperforations may be arranged such that the percentage reduction in airspeed is between 25% and 75% and more preferably between 30% and 70%. Inthe embodiment described below, the percentage reduction is between 40%and 60%. The percentage reduction may be less than 50%.

Preferably, the hole diameter and perforation percentage are chosen, sothat, at a typical landing approach speed, the air flowing through andbehind the airflow-reducing region is not turbulent, or at least, issuch that turbulence is low relative to the turbulence that would becaused downstream if the noise-reducing element did not include theairflow-reducing region.

The perforation percentage is preferably chosen such that, in use atnormal speeds on approach when landing, the airflow-reducing region hasa steady flow-resistance within the range 10 to 200 MKS Rayls (N.s.m⁻³)and more preferably within the range of 20 to 100 MKS Rayls.

The airflow-reducing region preferably covers an area, which would, ifthe airflow-reducing region were replaced with an extension of theair-deflecting surface, cover at least one stagnation point or cover atleast the majority of a stagnation line. The airflow-reducing region maybe so shaped as to only cover one or more stagnation points/lines,preferably all the stagnation points/lines, and the or each regionimmediately surrounding the or each stagnation point/line.Alternatively, the area covered by the airflow-reducing region may besignificantly greater than that required to cover the or each stagnationline/point. The coverage of the airflow-reducing region will of coursedepend not only on the shape and configuration of the noise-reducingelement, but also on the positioning of the noise-reducing element inrelation to the noise-generating parts of the landing gear/aircraft. Forexample, if the noise-reducing element is being used to shield a bogieundertray, control of the airflow downwards is not critical, whereasproper control of any airflows being deflected upwards will beimportant. Such a noise reducing element would benefit from anarrangement wherein the airflow-reducing region reduces the amount/speedof air deflected upwards.

The airflow in the vicinity of the stagnation point may under certainconditions be turbulent.

Of course, even with the provision of such one or more apertures,stagnation points may still occur. However, the presence of suchapertures near any such stagnation point may allow some air to escapethrough the aperture thereby reducing the flow velocity of the deflectedairflows, thus further reducing noise.

The or each aperture may be in the form of a gap, hole, passageway,opening or other means that allows air to flow through thenoise-reducing element as opposed to being deflected by it. It will beunderstood that, whilst preferred, the or each aperture need not forexample be bounded on all sides. For example, the apertures may belinked by elongate paths formed by other apertures. The or each apertureneed not be regular in shape. Indeed, one aperture may be so shaped toform a multiplicity of sub apertures.

The noise-reducing element is preferably so arranged that in use itshields at least a part of the landing gear. The noise-reducing elementis preferably in the form of a fairing that covers at least a part ofthe landing gear, when the landing gear is in a position in which it isable to support part of the weight of the aircraft on the ground. Saidpart of the landing gear may for example be a part of the landing gearthat has been identified as contributing to the generation of unwantednoise during landing. Said part of the landing gear may be in the regionof a steering column of a nose gear, a tow-bar, the underneath of abogie of a main landing gear, an articulated linkage, one or more rods,a brake actuator, a steering actuator, a door that in its closedposition covers the aperture through which the landing gear passes whenbeing deployed, and/or a dragstay.

More than one noise-reducing element may be provided on a single landinggear. Preferably, two or more noise-reducing elements are provided, eachnoise reducing-element including an air-deflecting surface and anairflow-reducing region through which, in use, air may pass. Eachadditional noise-reducing element may include any combination of thefeatures described above with reference to noise-reducing element of thelanding gear of the present invention. The or each noise-reducingelement may be formed of separate component parts. The or eachnoise-reducing element may alternatively be unitary in construction. Theair-deflecting surface may for example be a monolithic structure.

The present invention is of particular application on large aircraft,particularly passenger-carrying aircraft. For example, the landing gearis preferably of a size suitable for use on an aircraft designed tocarry more than 50 passengers, and more preferably more than 100passengers. Such aircraft generally have retractable landing gearassemblies.

The landing gear is preferably movable from a stored position to anoperative position.

The present invention also provides an aircraft including a landing gearaccording to any aspect of the above-described invention.

The present invention also provides a method of reducing noise caused bylanding gear on an aircraft including a step of manufacturing a landinggear according to any aspect of the above-described invention. Such amethod advantageously includes a step of modifying an existing design inorder to reduce noise caused by the landing gear.

There is also provided a noise-reducing element for use on an aircraftlanding gear, the noise-reducing element including an air-deflectingsurface and an airflow-reducing region through which, in use, air maypass, whereby the noise-reducing element is able in use to deflect airaway from a noise-generating region of the landing gear, whilst allowingsome air to pass through the element, thereby reducing the noise causedby the passing of the landing gear through the air. The reduction ofnoise may for example be effected by means of the reduction of thevelocity of deflected airflows.

The noise-reducing element may of course be so configured that it issuitable for use as the noise-reducing element of an aircraft landinggear according to any aspect of the above-described invention.

DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample with reference to the following schematic drawings of which:

FIG. 1 shows in perspective view a nose landing gear with fairings,

FIG. 2 shows a view of a region of a fairing of FIG. 1,

FIG. 3 shows in perspective view a main landing gear with fairings,

FIG. 4 shows an aircraft including both a nose landing gear as shown inFIG. 1 and two main landing gear as shown in FIG. 2, and

FIGS. 5 to 8 relate to a method of choosing the percentage perforationof a region of the fairing.

DETAILED DESCRIPTION

FIG. 1 illustrates a first embodiment of the invention concerning a noselanding gear 1. The nose landing gear 1 includes wheels 2 a centralsupport column 3 (or leg) and an upper fairing 4 and a lower fairing 5.The nose gear 1 is shown in its deployed position during landing of anaircraft (not shown in FIG. 1) to which the nose landing gear 1 isattached. The direction of flow of air relative to the nose gear isindicated by arrow A which points to the right in FIG. 1, since the nosegear and aircraft are moving to the left.

The upper fairing 4 is positioned over the central support column 3 in aposition that shields parts 6 of the landing structure that areassociated with the steering of the nose gear wheels 2. The parts 6 thatare shielded by the fairing include steering actuators (comprising rods,linkages and the like) that would if not shielded generate significantnoise. The upper fairing 4 is attached to the gear 1 via steeringactuator mounting brackets. The upper fairing 4 has regions of bothpositive and negative curvature, and in the region at the front of thefairing, the fairing is saddle-shaped.

The lower fairing 5 is positioned over and therefore shields a tow-bar(hidden from view in FIG. 1) and jacking structure. The tow-bar, whichis positioned between the fairing 5 and the central support member 3,may be used to tow the aircraft when the aircraft is on the ground andthe jacking structure enables the aircraft to be jacked up by its nosegear to enable the nose wheels 2 to be serviced. Access to the tow barand jacking structure is facilitated by means of a door (not shown inFIG. 1) in the lower fairing 5. The tow bar and jacking structure wouldalso, if not shielded, generate significant noise. The lower fairingdoes not have any regions having a curvature resembling a saddle.

Each fairing 4, 5 has an aerodynamically-streamlined profile shaped sothat the air approaching the fairing is deflected by the fairing awayfrom the part of the landing gear that it shields. The fairings aregenerally convex in shape as viewed in the direction of arrow A and aremade from sheet aluminium having a thickness of 1.6 mm.

Each fairing also has a perforated region 8 indicated by the shadedregions in FIG. 1. The perforated region is shown schematically in planview in FIG. 2. The region shown in FIG. 2 is a region of perforationsformed in a flat plane (the plane being flat for the sake of clarity);it will of course be appreciated that most, if not all, of theperforated regions 8 on the fairings 4,5 are non-planar. Theperforations are in the form of round holes formed in the fairing andare arranged in a square matrix of notional unit cells 10, each unitcell having a perforation in its centre. The cross-sectional area of aperforation varies along its length. The area decreases substantiallyconstantly with length from the front face (in the direction of travelof the fairing) to a given depth (for example between ¼ and ⅓ of thelength of the hole that defines the perforation—i.e. the thickness ofthe fairing). After that given depth the cross-sectional area of theperforation is substantially constant up to the back face. Theperforations are thus in the form of countersunk holes, which are easilyand readily formed in a blank fairing (or one or more sheets ofaluminium to be made into a fairing) by drilling.

The diameter of each hole on the front face is 4.3 mm. The diameterreduces to 3.5 mm at a depth from the front surface of 0.4 mm (i.e. thehole flares out at an angle of 45°). The fact that the holes arecountersunk on the front face means that there are no sharp edgespresented to the incident airflow. If the holes were not countersunk,the sharp edges of the holes on the front face could cause oscillationsin the airflow, thereby generating unwanted noise.

The notional unit cells 10 of the perforated region 8 have a width andheight of 4.7 mm. The perforations each have a diameter of 3.5 mm attheir narrowest. Thus adjacent perforations are separated by 1.2 mm(their centres being separated by 4.7 mm). The percentage of the area ofthe perforated region that is perforated (taking the minimumcross-sectional areas of the perforations) is 44%, the remaining 56%being formed of solid material (the same material as the rest of thefairing). The percentage of the area of the perforated region that isperforated (taking the maximum cross-sectional areas of theperforations) is 66%.

The perforated regions 8 are so shaped and positioned on the fairingsthat they do not extend all the way to the edge of the fairing but docover the point, which if the fairing were not perforated in theperforated region, would form a stagnation point.

The term stagnation point is well known in the art. By way of example,and in relation to the present embodiment, the term may simply bedefined as the point on the fairing at which it may be considered theair impacting against the fairing divides. Alternatively, as a verysimple approximation, the stagnation point on a surface that istravelling in a given direction may be considered as being the point atwhich the normal to the surface is parallel to the direction of travel.Thus a given surface may have several stagnation points or the surfacemay have an infinite number of stagnation points, forming a stagnationline or even a stagnation area. Also, the position of the or eachstagnation point will vary with the direction of relative motion betweenthe surface and the fluid through which it moves.

The fairings are of course adequately vented so as not to prevent orhinder the flow of air through the perforations. In particular, thestructure is so shaped, in relation to the rest of the aircraftdownstream, that a given unit volume of air that flows through theperforations in the perforated region does not pass through an effectiveconstriction downstream having an area smaller than the effective areaof constriction of the perforated region through which the air passed.The fairing is in particular not a closed fairing.

On landing the aircraft, the speed of the air relative to the noselanding gear will for example be about 85-90 ms⁻¹ (i.e. roughly equal tothe ground to air speed). During the approach, when the nose gear 1 hasbeen deployed, the air flowing towards the fairings 4, 5 is partlydivided and deflected away from the components of the landing gear thatthe fairings shield. However, some of the air passes (or “bleeds”)through the fairings 4, 5 via the perforations 9 in the perforatedregions 8. The air that passes through the fairings 4, 5 in this way isslowed down, by means of the friction between the fairing in theperforated region 8 and the air. The speed of the air relative to thefairing 4, 5 on the rear side of the fairing immediately behind theperforated regions 8 might for example be about 40% to 50% of the speedof the air relative to the fairing in front of the nose gear 1. Thenoise generated by the passing of this air over the components that areotherwise shielded by the fairings is less than the extra noise thatwould be generated by the airflows deflected by the fairing had theperforations not been provided in the fairing.

The countersinking of the perforations helps reduce the chances ofresonance of the fairing at certain conditions which could cause extraunwanted noise at certain frequencies.

A second embodiment of the invention is shown in FIG. 3, which shows amain landing gear 11 including wheels 2 and a central support column 3(or leg). The gear 11 includes several fairings 12, 13, 14, 15. The maingear 11 is shown in its deployed position during landing of an aircraft(not shown in FIG. 3) to which the main landing gear 11 is attached. Thedirection of flow of air relative to the landing gear is indicated byarrow B which points to the left in FIG. 3, since the main landing gearand aircraft are moving to the right.

The fairings illustrated in FIG. 3 include an undertray fairing 12, anarticulation-link fairing 13, a door/dragstay-closure fairing 14 and anupper side-stay fairing 15. Each fairing includes, in a manner similarto that of the fairings shown in FIG. 1, a perforated region 8 whichcovers a stagnation point or part of a stagnation line. Thus theprinciples behind and improvements provided by the arrangement of thefairings shown in FIG. 3 and the perforated regions 8 thereon are thesame as those described with reference to FIG. 1.

The undertray fairing 12 is a large single curvature cover that shieldsthe underneath of the main landing gear bogie and brake rod area. Thefairing 12 includes a forward-facing raised area 16 that has astreamlined shape which thus presents a smoother surface to the air thanthe landing gear would present without the fairing 12. The fairing 12 isclamped to both the bogie beam and the axles (not shown clearly in FIG.3). The fairing 12 includes flush, removable doors (not shown for thesake of clarity in FIG. 3) that allow ready access to the undercarriagejacking structure and access to facilitate servicing of the wheels 2.

The articulation-link fairings 13 include an upper fairing 13 a and alower fairing 13 b. The fairings 13 a, 13 b together cover and shieldthe gear-articulation-link mechanisms and jack. The fairings 13 a, 13 bare both clamped to their respective articulation links and are joinedby an aerodynamic seal indicated by joint 17 in FIG. 3. Also thepositioning of the lower-articulation-link fairing 13 b is such that itslower end co-operates with the upper surface of the undertray fairing12, when the bogie of the main landing gear is positioned in the trimmedposition for landing (the position shown in FIG. 3).

The door/dragstay-closure fairing 14 shields the landing gear in theregion between the dragstay 20 and the inner surface 18 of the mainlanding gear door 19. The fairing is shaped to provide anaerodynamically-smoother profile than would be provided if the forwardflat surface 21 of the dragstay 20 were unfaired. The fairing 14 is alsoshaped such that it allows the side stay 21 to fold into the fairing 14when the landing gear 11 is retracted and stowed.

The upper side-stay fairing 15 shields the otherwise flat surface of theupper part of the side stay 21 and again provides a moreaerodynamically-acceptable surface than the bare sidestay 21. Thefairing 15 also covers the gear actuation springs (not shown in FIG. 3).

Each fairing, in a manner similar to the first embodiment, is made fromaluminium sheet material. The perforated regions 8 are similar to thoseof the perforated regions described with reference to the firstembodiment, those regions being illustrated schematically by FIG. 2.

FIG. 4 shows a third embodiment of the present invention relating to anaircraft 22. The aircraft 22 includes a nose landing gear 1 inaccordance with the first embodiment and two sets of main landing gears11 in accordance with the second embodiment of the invention (only oneset of main landing gear 11 being shown in FIG. 4 for the sake ofclarity). The aircraft 22 having landing gears that are provided withfairings having perforated regions as described above may result in thenoise generated by the aircraft on its approach when landing beingsubstantially reduced.

It will be readily apparent to the skilled person that variousmodifications may be made to the above-described embodiment withoutdeparting from the spirit of the invention. For example, fewer orgreater fairings may be provided and/or the function provided by two ormore fairings may be provided by a single fairing if possible.

The cross-sectional area of each perforation need not vary along itslength through the fairing and could instead be substantially constant.Also, the perforation could include a portion that flares from a givendepth to the rear face of the fairing, so that the perforations are inthe form of holes that are countersunk on both faces of the fairing.

Other patterns of holes could be used. For example, the perforations maybe arranged in a matrix, where each perforation if formed in the centreof a notional unit cell in the shape of a regular hexagon. An irregulararrangement of perforations may even be implemented.

The diameter, and spacing of the perforations (and consequently thepercentage of the perforated region that is perforated) may be adjustedto suit a particular aircraft and/or to suit a chosen range ofaircraft-to-ground speeds on landing.

For example, the optimum percentage of perforation is thought to bedependent on the relative speed of the fairing and the air. Differentsize and shape of aircraft, and different landing speeds, may thereforeaffect the ideal percentage of perforation. One method of choosing theperforation percentage is described below, purely by way of exampleonly, with reference to FIGS. 5 to 8.

FIG. 5 shows a perforated plate in a flow stream of velocity u₁, FIG. 6shows a graph of downstream to upstream velocity ratio as a function ofDC flow resistance and upstream velocity, FIG. 7 shows a graph ofu-component of the velocity distribution across scan plane for ahalf-cylinder perforated shell as measured in a wind tunnel test (onlyhalf of the measurements data points being shown since they areapproximately symmetric), and FIG. 8 shows a graph of predicted flowdistribution across scan plane for u₁=90 m/s, and 3 different open-areacoefficients.

The publication Massey, B. S. Mechanics of Fluids. 5^(th) ed. VanNostrand Reinhold(UK), 1983 addresses the problem of a jet of fluidincident at an angle θ to a rigid surface, assuming inviscid,incompressible and irrotational flow. A similar approach is followed forthe current problem but the surface is now perforated so that some fluidcan flow through it. This situation is shown in FIG. 5.

The rate at which momentum enters the control volume enclosed by S inthe direction perpendicular to the plate is∫ρu ₁ u _(1x) dA=ρAu ₁ ² cos θand the rate at which it leaves the volume in the x direction is∫ρu ₂ u _(2x) dA=ρAu ₂ ² cos θwhere u_(ix)=u_(i) cos θ is the velocity component in the x direction.By Newton's second law, the excess in momentum rate in the x directionacross the surface S is equal to a force F_(x) on the perforate surfacesuch thatF _(x)=(p ₂ −p ₁)A=−ρA cos θ(u ₂ ² −u ₁ ²)  (1)According to the publication Ingard, K. U. Notebook #3 Notes On SoundAbsorption Technology. Ver94-02. Noise Control Foundation, N.Y., 1994for a thin perforated surface the (velocity dependent) steady flowresistance r_(DC) is given by:

$\begin{matrix}{{r_{D\; C}\left( u_{or} \right)} \equiv \frac{\Delta\; p}{u_{or}}} & (2)\end{matrix}$where ΔP is the pressure drop across the plate and u_(or) is the meanflow velocity through the orifices. We assume here that for a perforatedplate of open-area coefficient σ the velocity in the orifices u_(or) canbe approximated by

$u_{or} \approx \frac{u_{2}}{\sigma}$

Multiplying eq. (1) by eq. (2) to give a quadratic equation in u₂,choosing the solution with the positive square root and rearranginggives

$\begin{matrix}{{\frac{u_{2}}{u_{1}}\left( {r_{D\; C},u_{1},\theta} \right)} \approx {{\Omega\left( {r_{D\; C},u_{1},\theta} \right)}\left( {\sqrt{1 + \frac{1}{{\Omega\left( {r_{D\; C},{u_{1}\theta}} \right)}^{2}}} - 1} \right)}} & (3)\end{matrix}$where

${\Omega\left( {r_{DC},u_{1},\theta} \right)} = {\frac{r_{DC}}{2u_{1}\sigma\;\rho\;\cos\;\theta}.}$

FIG. 6 shows u₂/u₁ as a function of r for different incidence velocitiesand normal incidence (θ=0°).

The calculations made here are for a perforated plate, the perforationsbeing arranged in a hexagonal matrix and having a pitch p=5.0 mm andorifice diameter d_(or)=3.5 mm. For the hole pattern of this materialthe porosity of the material is given by:

$\sigma \approx \left( {{.95}\frac{d_{or}}{p}} \right)^{2} \approx {0.44.}$The free stream velocity in the wind tunnel was u₁=60 m/s, and weapproximate the velocity in the orifices by

$u_{or} = {\frac{u_{1}}{\sigma} \approx {136\mspace{14mu} m\text{/}s}}$giving a Reynolds number

${Re} = {\frac{u_{or}d_{or}}{v} \approx {3.1 \times 10^{4}}}$which, according to the publication Idelchick, I. E. Handbook OfHydraulic Resistance, 2^(nd) ed. Hemisphere Publishing Corp. 1986determines the flow across the perforate to be in a transitional region,between laminar and fully turbulent flow. Thus using diagram 8-5 of theIdelchick reference we calculate the resistance coefficient

$\zeta = {\frac{\Delta\; p}{{1/2}\;\rho\; u_{1}^{2}} \approx {{\zeta_{\phi}\frac{1}{\sigma^{2}}} + {{\overset{\_}{ɛ}}_{0{Re}}\zeta_{1{qu}}}}}$where ζ_(φ)=f(Re,σ) accounts for ‘laminar’ viscous losses, ε _(0Re) is afactor Reynolds number dependant, and ζ_(1qw) is the resistancecoefficient for fully turbulent flow (Re>10⁵). The above are read from agraph or table to give

$\zeta \approx {{0.02\frac{1}{0.45^{2}}} + {0.82 \times 6.45}} \approx 5.4$In order to relate this resistance coefficient with the DC flowresistance defined above we can write

$\zeta = {{\frac{\Delta\; p}{{1/2}\;\rho\; u_{1}^{2}} \approx \frac{\Delta\; p}{{1/2}{\rho\left( {u_{or}\sigma} \right)}^{2}}} = {\frac{2r_{D\; C}}{\rho\; u_{or}\sigma^{2}} \approx \frac{2r_{D\; C}}{\rho\; u_{2}\sigma}}}$and rearranging and assuming u₂≈0.5u₁

$r_{D\; C} \approx {\frac{1}{2}\zeta\;\rho\; u_{2}\sigma} \approx {43{{{ray1}s}({MKS})}}$

FIG. 6 confirms that for u₁=60 m/s and a DC flow resistancer_(DC)≈43rayl(MKS), the predicted velocity u₂ downstream of theperforated plate should indeed be around half of u₁, in factu₂≈0.54u₁≈32.4m/s. If this was not the case, further guesses for u₂would have to be made and the last steps iterated until agreement isachieved. FIG. 7 compares the prediction of eq. (3) as a function ofangle θ with some data points read from the results of the measurementson the perforated plate. The agreement is good up to angle θ=±25°, afterwhich the assumption of negligible mass flow tangential to the plate isno longer acceptable.

The former calculation is now reversed in order to (tentatively) specifythe perforated plate for flight conditions. Because of the non-linearequations involved it is necessary to use an iterative procedure.Setting

u₁ = 90  m/s $\frac{u_{2}}{u_{1}} = 0.5$ d_(or) = 3.5  mmthen, from FIG. 6 read r_(DC)≈72rayl, and an initial trial guess forσ₀=0.35 gives

$\zeta \approx \frac{2r_{DC}}{\rho\; u_{2}\sigma} \approx 7.62$The Reynolds number is

${Re} = {\frac{\frac{90}{0.35} \times 3.5 \times 10^{- 3}}{15.1 \times 10^{- 6}} \approx {6 \times 10^{4}}}$and from diagram 8-5 in of the Idelchick reference

${\left. \begin{matrix}{\;{{\overset{\_}{ɛ}}_{0{Re}} \approx 0.91}} \\{\zeta_{\phi} \approx 0.02}\end{matrix} \right\}\zeta_{1{qu}}} \approx \frac{\zeta - \frac{\zeta_{\phi}}{\sigma^{2}}}{\;{\overset{\_}{ɛ}}_{0{Re}}} \approx {8.2.}$Finally, from diagram 8-1 in of the Idelchick reference we can read thefree-area coefficient corresponding to this resistance coefficient:ζ_(1qw)=8.2→σ≈0.39which is not in agreement with our initial guess.If we try as a second iteration for σ₁=0.42, the same calculation givesthe correct valueζ_(1qw)≈6.8→σ≈0.42

FIG. 8 compares the predicted velocity distribution from eq. (3) for aperforate plate with σ0.35, σ=0.42, and σ=0.44 for the flight conditionsspecified.

Since the diameter of the orifices was fixed at 3.5 mm, an assuming astaggered arrangement of holes, the pitch required to give each porosityis

${pitch} = {p = {\frac{0.95d_{or}}{\sqrt{\sigma}} = \left\{ \begin{matrix}{5.62\mspace{14mu}{mm}} & \Leftarrow & {\sigma = 0.35} \\{5.13\mspace{14mu}{mm}} & \Leftarrow & {\sigma = 0.42} \\{5.01\mspace{14mu}{mm}} & \Leftarrow & {\sigma = 0.44}\end{matrix} \right.}}$

From FIG. 7 it is concluded that the model is in reasonable agreementwith the data measured.

From FIG. 8 it is concluded that, assuming a hole size of 3.5 mm, theappropriate percentage open area for the flight condition is in therange 42%-44%. For the staggered hole arrangement of the original testsample (to be specified in detail) the appropriate hole pitch is thus5.01 mm-5.13 mm. This specification is virtually identical to thematerial tested in wind tunnel tests.

The method described above with reference to FIGS. 5 to 8 is provided byway of example only to illustrate the issues that preferably need to beconsidered when choosing the arrangement and size of the perforations.It will of course be appreciated that other methods could be employed tospecify the perforation pitch and diameter, and therefore theaerodynamic porosity. For example, wind tunnel tests and/or computersimulations could be utilised to obtain acceptable values by trial anderror.

Adjacent perforations could be connected by long and thin apertures suchthat a plurality of perforations could be considered as forming asingle, albeit complexly-shaped, hole.

The fairing could be made from materials other than aluminium or alloysthereof. For example, the fairing could be made from carbon fibre orglass fibre composite materials or even plastic material.

It is also thought that the provision of a mesh might provide evenbetter results in terms of noise reduction. The mesh would be providedin front of the fairing and would, in particular, cover the region thatis perforated. The gauge of the mesh would be much finer than thediameter of the perforations. For example, it is envisaged that a gapsize in the mesh of 0.1 mm to 2 mm would be preferable. The mesh wouldfurther enhance the effect of the fairing of reducing the air velocitybehind the fairing whilst letting some air through. The mesh could bearranged as a renewable item and as such would be removably mounted inrelation to each fairing. A wire mesh would be suitable. The thicknessof the wire or material forming the mesh should be as low (thin) aspossible, but must of course be great enough for the mesh to be able tocope with the harsh environment (i.e. high air velocities) that the meshwould be subjected to on landing. The thickness could be between 0.01 mmto 1 mm. A thickness of less than 0.1 mm is preferable. The dimensionsof the mesh may be similar to those of a conventional mosquito net. Itis preferred that the mesh has a flow resistance, at approach speeds, ofless than 200 MKS Rayls and preferably in the range of 10 to 100 MKSRayls.

Of course, the invention is applicable to all aircraft where unwantednoise is an issue. As such, the present embodiment could of course beapplied to aircraft having landing gear arrangements different fromthose described above with reference to the accompany drawings.

1. An aircraft landing gear including a noise-reducing element thatincludes an air-deflecting surface wherein said air-deflecting surfaceincludes a first region encompassing no apertures and a second regionencompassing at least ten apertures, the area covered by the firstregion having a minimum dimension that is at least as great as themaximum dimension of the area covered by the second region, and anairflow-reducing region having more than 10 apertures through which, inuse, air may pass, whereby the noise-reducing element is able in use todeflect air away from a noise-generating region of the landing gear,whilst allowing some air to pass through the element, thereby reducingthe noise caused by the passing of the landing gear through the air. 2.An aircraft landing gear according to claim 1, wherein the 15noise-reducing element includes at least 50 apertures.
 3. An aircraftlanding gear according to claim 1, wherein the apertures are in the formof perforations.
 4. An aircraft landing gear according to claim 1,wherein the arrangement of the apertures across the air-deflectingsurface is non-uniform.
 5. An aircraft landing gear according to claim1, wherein the first region is near to the periphery of theair-deflecting surface.
 6. An aircraft landing gear according to claim1, wherein the airflow-reducing region is disposed between two regionsdefined by the air-deflecting surface.
 7. An aircraft landing gearaccording to claim 1, wherein the airflow-reducing region covers anarea, which would, if the airflow-reducing region were replaced with anextension of the air-deflecting surface, cover at least one stagnationpoint or cover at least the majority of a stagnation line.
 8. Anaircraft landing gear according to claim 1, wherein the noise-reducingelement is so arranged that in use it shields at least a part of thelanding gear.
 9. An aircraft landing gear according to claim 1, whereinthe noise-reducing element is in the form of a fairing that covers atleast a part of the landing gear.
 10. An aircraft landing gear accordingto claim 1, wherein the landing gear is movable from a stored positionto an operative position.